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Barrel Gas Port Size,308/338/260/243 etc.


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It's the volume of the actual gas port.  It's taken a very, very long to to learn this, and apply it.  Once of these days, I'll give it up.  I've given up pieces of it in the past.  All those gas port ranges that I come up with, that work - vary by dwell time.  You need to determine what works, based on dwell time, then compute the gas port volume.  After that, you can compute what works for different calibers.  It ain't easy, and it's taken alot of ammo money to realize that I could have been doing it easier, with less money.  Just apply math.  Nothing can beat the math.  That's unpossible. 

This isn't Terminator T1000.  It's just a mechanical device.  It can't out-think us.  :thumbup:

Edited by 98Z5V
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Many years ago, here on the board, we had a very well known buffer manufacturer come on here and rip us apart, telling us we didn't know anything about recoil systems - based on some whiner on another message board complaining about us. 

That was a defining moment for me, and I determined that I would be the smartest motherfucker on the planet on recoil systems, after that.  But that went sideways, because there's another part of recoil systems.  Gas systems.  If you don't balance those two systems, your rifle will not work properly. 

So, in a nutshell, I delved into recoil systems, gas systems, learned everything I could online (people's posted problems), used my own research and information on what I'd built and done, and really started understanding it.  At that point, the lightbulb went on - this is all math...  Nothing more.

I have over 40 ARs total, in 12 or 13 different calibers.  It was beneficial to me to learn this, and not just throw away ammo money figuring it out the hard way.  Based on that experience, that gives me the ability to inquire about someone's build, ask specific questions about the details on their parts selection - and be able to tell them if their gun is gonna work - or need work, to work.  It's been a long journey, but worth it.  I have no idea how many guns I've fixed over the internet, just from typed words, when given the proper information to do so - but it's in the hundreds, just right here on this board.

I'm not Eugene Stoner - I just study what he developed.  Every little detail matters.

Read this please, when you have the time - it'll give you a deeper understanding of why this is all important:

 

Edited by 98Z5V
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11 minutes ago, 98Z5V said:

in 12 or 13 different calibers. 

14 calibers, unless I forgot one.  That does happen.  I forget about some guns, sometimes.  Not my fault, really.  These are just the AR calibers.  This doesn't include shotguns, bolt guns, pistols, etc.  I reload for 22 or 23 calibers, IIRC.  Have to look at the die stack to be sure. 

6 arc
6.5 grendel
224 valk
9mm
45 acp
22lr
5.56
300blk
25/45 sharps
308 win
260 rem
338 fed
358 win
7mm-08

Edited by 98Z5V
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14 hours ago, 98Z5V said:

Many years ago, here on the board, we had a very well known buffer manufacturer come on here and rip us apart, telling us we didn't know anything about recoil systems - based on some whiner on another message board complaining about us. 

That was a defining moment for me, and I determined that I would be the smartest motherfucker on the planet on recoil systems, after that.  But that went sideways, because there's another part of recoil systems.  Gas systems.  If you don't balance those two systems, your rifle will not work properly. 

So, in a nutshell, I delved into recoil systems, gas systems, learned everything I could online (people's posted problems), used my own research and information on what I'd built and done, and really started understanding it.  At that point, the lightbulb went on - this is all math...  Nothing more.

I have over 40 ARs total, in 12 or 13 different calibers.  It was beneficial to me to learn this, and not just throw away ammo money figuring it out the hard way.  Based on that experience, that gives me the ability to inquire about someone's build, ask specific questions about the details on their parts selection - and be able to tell them if their gun is gonna work - or need work, to work.  It's been a long journey, but worth it.  I have no idea how many guns I've fixed over the internet, just from typed words, when given the proper information to do so - but it's in the hundreds, just right here on this board.

I'm not Eugene Stoner - I just study what he developed.  Every little detail matters.

Read this please, when you have the time - it'll give you a deeper understanding of why this is all important:

 

For me, it was seeing posts every day about how "AR-10's are so hard to tune," "You need an adjustable gas block for a large frame rifle," "building an LR308 sucks and mine wouldn't run for anything so you should just buy a complete one." I just wasn't willing to accept that answer, so I set out on my first to prove them wrong.

About a year ago, I started researching gas dynamics and how it pertained to the direct impingement gas system. I had hoped that I could simplify the whole system as a dynamic equation since it really is just a multiple spring dampened mass problem. All that needed to be solved was the initial force and duration of the gas on the carrier. As a simplified system, I didn't figure it would be perfect without a lot of refining, but it would probably get me close.

That researching did lead me here, where I found the legwork done for me on so many common calibers and configurations. I still have dreams of completing that work, more for my own amusement than anything, but also to help other people online see that the large frame rifles aren't THAT hard when done correctly. Maybe they have to be a bit more meticulous in their research and parts selection than with an AR-15, but they don't require an adjustable gas block and they sure as hell can be tuned reliably.

And look at that: my first one went off without a hitch. Even less problems putting it together and running than my AR-15 did.

So I applaud you, sir, for your effort and contribution to the ones willing to listen.

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9 hours ago, MarkLeupold said:

For me, it was seeing posts every day about how "AR-10's are so hard to tune," "You need an adjustable gas block for a large frame rifle," "building an LR308 sucks and mine wouldn't run for anything so you should just buy a complete one." I just wasn't willing to accept that answer, so I set out on my first to prove them wrong.

^^^  That's the truth, right there.  The whole statement, but the part specifically about adjustable gas blocks.  I don't own a single adjustable gas block for any AR that I have.  Not one single one.  Get the gun to run, and you don't need one. Just figuring a caliber out is why I have so many weird calibers in these things. 

Balance the Gas System to the Proper Recoil System for the large frames, because that's the heart of your reliability, right there. 

Barrel configuration and caliber play a big part of that - but we've pretty much got all that figured out by now, from examples here on the board, and getting those guns running, just over the years. 

Edited by 98Z5V
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New build is short stroking.  It ejects about 1' @ 3 o'clock and will not lock bolt back on empty mag or strip off next round.  .308, 20" Shilen barrel, rifle length gas system, .75 gas mandrel, JP adjustable gas block (wide open) 5.6 oz rifle buffer.  I suspect the gas port is too small at .076 (caliper measurement).  I am planning on buying numbered bits tomorrow to confirm size and possibly open it incrimentally until it runs.  Where should it be .093 - .096?

Also, I have only shot 50 rounds through it and I suspect the problem could be contributed to the fact that I am loading 130 grain Barns TTSX, which is very light for caliber but they are near max load at 3250 fps.  That gives a power factor of 422 vs 175 gr bullets @ 2600 fps (PF 455) which is the Federal load that Shilen uses as a proof load.  I know the easy answer to this question is to try heavier factory ammo, but is it likely that the 130 gr bullet is the cause of the cycle failure?  If so is it reasonable to attempt to tune for this load?  I really want to use a fast monolithic bullet for hog hunting at night wich is the primary reason for this build.

First post.... and thank you for any feed back!

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10 minutes ago, LorinB said:

First post.... and thank you for any feed back!

Start a build thread on it - this isn't the section nor the thread for it.

Here's the build section - start your post in here with the same information:

https://forum.308ar.com/forum/47-building-a-308ar/

Welcome aboard.  :thumbup:

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9 hours ago, Destructor345 said:

My new 20" Lothar-Walther barrel has a .080 gas port, .875 journal, and 15.25" tube (the 15.5" Wilson tube was too long).  The gas port is just over 7 1/16" from the muzzle.  I would like to use it with a 5.3oz buffer and Sprinco orange spring.  Could you please tell me the gas port size it should be?

 

7 hours ago, Destructor345 said:

Oh I forgot to add, the barrel above is chambered for .308 Win.

Start a build thread on your rifle.  This section isn't really the question-answer section, it's just to report info.

Here's the build section:

https://forum.308ar.com/forum/47-building-a-308ar/

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  • 2 months later...
On 1/22/2023 at 1:29 PM, 98Z5V said:

Wilson Combat Tactical Hunter barrel in 7mm-08 caliber.  18" Rifle Gas, 0.750" gas block journal diameter.  Ships with a gas port that 0.076".

I'll check some math and see if this is gonna work. 

So, I knew right away that port diameter was way low.

On 1/26/2023 at 8:04 PM, 98Z5V said:

No possible way this 0.076" gas port will work.  Will never happen, with a proper recoil system in place.  It's gonna have to be 0.085" minimum, and probably 0.090" in the end.  I think it's just gonna get the 0.090" treatment right away.  As it is, it's just not right. 

Thanks again, WC.  :thumbup:

After running and looking at numbers, it ended up going 0.090" port diameter before the upper was ever built.  It went out, was shot with that 0.090" port, and had a very first round, single round lockback, with light lube on the BCG and upper, buffer, spring, internal lube on the receiver extension.  Not with normal lube, with the special stuff.

Bottom line - the math works.  The gun runs, and runs well.  Now, the load needs the work, and that one could be a winner...  

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  • 1 month later...

Any math on  this one?

Faxon Duty Series 16" Big Gunner, 8.6 Blackout

Port .1050"

Specs:

  • Barrel Material: 4150 CMV
  • Rifling: Button
  • Barrel Caliber: 8.6 BLK
  • Barrel Twist: 1:3
  • Barrel Length: 16"
  • Barrel Profile: Big Gunner
  • Barrel Gas System: Carbine
  • Inside Finish: Salt Bath Nitride
  • Outside Finish: Salt Bath Nitride
  • Muzzle Thread: M18x1.5, tapered shoulder (adapter included)
  • Gas Block Diameter: .875"
  • Gas Block Journal Length: 1.9"
  • Barrel Extension: Standard DPMS Gen 1 .308 Extension, Black Oxide
  • Weight: 2.2 lbs
  • Magnetic Particle Inspected
  • 11° Target Crown
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On 5/27/2023 at 5:48 AM, Turd Furgeson said:

Any math on  this one?

Faxon Duty Series 16" Big Gunner, 8.6 Blackout

Port .1050"

Specs:

  • Barrel Material: 4150 CMV
  • Rifling: Button
  • Barrel Caliber: 8.6 BLK
  • Barrel Twist: 1:3
  • Barrel Length: 16"
  • Barrel Profile: Big Gunner
  • Barrel Gas System: Carbine
  • Inside Finish: Salt Bath Nitride
  • Outside Finish: Salt Bath Nitride
  • Muzzle Thread: M18x1.5, tapered shoulder (adapter included)
  • Gas Block Diameter: .875"
  • Gas Block Journal Length: 1.9"
  • Barrel Extension: Standard DPMS Gen 1 .308 Extension, Black Oxide
  • Weight: 2.2 lbs
  • Magnetic Particle Inspected
  • 11° Target Crown

Is it working, or not?  Since this is really an information thread on what works, instead of a thread to ask questions, just start a thread on your gun in the build sections (either one, there are two areas), and post the same information, along with what it's doing right now when you shoot it.  Thanks in advance.  :thumbup:

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27 minutes ago, 98Z5V said:

Is it working, or not?  Since this is really an information thread on what works, instead of a thread to ask questions, just start a thread on your gun in the build sections (either one, there are two areas), and post the same information, along with what it's doing right now when you shoot it.  Thanks in advance.  :thumbup:

Haven't had time to get out yet, but will  do.  More to come, thanks.

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  • 2 weeks later...

Don't know if I've entered this one here or not before, but I should have. 

Ballistic Advantage .223 Wylde chamber, 1:8" twist, 16" SPR-profile barrel with Midlength Gas system.  0.750" gas block journal diameter, 0.076" gas port diameter.   It'll do just fine with that gas port.   :thumbup:

This one here:

BA16SPR.thumb.jpg.6b3c5c82fd6bb982c984beb9f8cea77c.jpg

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